Magnetic sensor having a seedlayer for providing improved hard magnet properties

ABSTRACT

A magnetoresistive sensor having a novel seed layer that allows a bias layer formed there over to have exceptional hard magnetic properties when deposited over a crystalline structure such as an AFM layer of in a partial mill sensor design. The seed layer structure may be a CrMo/Si/CrMo sandwich or may also be a CrMo/Si/Cr sandwich and interrupts the epitaxial growth of an underlying crystalline structure, allowing a hard magnetic material formed over the seed layer to have a desired grain structure that is different from that of the underlying crystalline layer.

FIELD OF THE INVENTION

The present invention relates to free layer biasing in amagnetoresistive sensor, and more particularly to a seed layer for usewith a bias layer formed over a crystalline structure.

BACKGROUND OF THE INVENTION

The heart of a computer's long term memory is an assembly that isreferred to as a magnetic disk drive. The magnetic disk drive includes arotating magnetic disk, write and read heads that are suspended by asuspension arm adjacent to a surface of the rotating magnetic disk andan actuator that swings the suspension arm to place the read and writeheads over selected circular tracks on the rotating disk. The read andwrite heads are directly located on a slider that has an air bearingsurface (ABS). The suspension arm biases the slider toward the surfaceof the disk and when the disk rotates, air adjacent to the surface ofthe disk moves along with the disk. The slider flies on this moving airat a very low elevation (fly height) over the surface of the disk. Thisfly height is on the order of Angstroms. When the slider rides on theair bearing, the write and read heads are employed for writing magnetictransitions to and reading magnetic transitions from the rotating disk.The read and write heads are connected to processing circuitry thatoperates according to a computer program to implement the writing andreading functions.

The write head includes a coil layer embedded in first, second and thirdinsulation layers (insulation stack), the insulation stack beingsandwiched between first and second pole piece layers. A gap is formedbetween the first and second pole piece layers by a gap layer at an airbearing surface (ABS) of the write head and the pole piece layers areconnected at a back gap. Current conducted to the coil layer induces amagnetic flux in the pole pieces which causes a magnetic field to fringeout at a write gap at the ABS for the purpose of writing theaforementioned magnetic impressions in tracks on the moving media, suchas in circular tracks on the aforementioned rotating disk.

In recent read head designs a spin valve sensor, also referred to as agiant magnetoresistive (GMR) sensor, has been employed for sensingmagnetic fields from the rotating magnetic disk. This sensor includes anonmagnetic conductive layer, hereinafter referred to as a spacer layer,sandwiched between first and second ferromagnetic layers, hereinafterreferred to as a pinned layer and a free layer. First and second leadsare connected to the spin valve sensor for conducting a sense currenttherethrough. The magnetization of the pinned layer is pinnedperpendicular to the air bearing surface (ABS) and the magnetic momentof the free layer is biased parallel to the ABS, but is free to rotatein response to external magnetic fields. The magnetization of the pinnedlayer is typically pinned by exchange coupling with an antiferromagneticlayer.

The thickness of the spacer layer is chosen to be less than the meanfree path of conduction electrons through the sensor. With thisarrangement, a portion of the conduction electrons is scattered by theinterfaces of the spacer layer with each of the pinned and free layers.When the magnetizations of the pinned and free layers are parallel withrespect to one another, scattering is minimal and when themagnetizations of the pinned and free layer are antiparallel, scatteringis maximized. Changes in scattering alter the resistance of the spinvalve sensor in proportion to cos θ, where θ is the angle between themagnetizations of the pinned and free layers. In a read mode theresistance of the spin valve sensor changes proportionally to themagnitudes of the magnetic fields from the rotating disk. When a sensecurrent is conducted through the spin valve sensor, resistance changescause potential changes that are detected and processed as playbacksignals.

When a spin valve sensor employs a single pinned layer it is referred toas a simple spin valve. When a spin valve employs an antiparallel (AP)pinned layer it is referred to as an AP pinned spin valve. An AP spinvalve includes first and second magnetic layers separated by a thinnon-magnetic coupling layer such as Ru. The thickness of the spacerlayer is chosen so as to antiparallel couple the magnetic moments of theferromagnetic layers of the pinned layer. A spin valve is also known asa top or bottom spin valve depending upon whether the pinning layer isat the top (formed after the free layer) or at the bottom (before thefree layer).

Magnetization of the pinned layer is usually fixed by exchange couplingone of the ferromagnetic layers (AP 1) with a layer of antiferromagneticmaterial such as PtMn. While an antiferromagnetic (AFM) material such asPtMn does not in and of itself have a magnetic moment, when exchangecoupled with a magnetic material, it can strongly pin the magnetizationof the ferromagnetic layer.

The spin valve sensor is located between first and second nonmagneticelectrically insulating read gap layers and the first and second readgap layers are located between ferromagnetic first and second shieldlayers. In a merged magnetic head a single ferromagnetic layer functionsas the second shield layer of the read head and as the first pole piecelayer of the write head. In a piggyback head the second shield layer andthe first pole piece layer are separate layers.

As those skilled in the art will appreciate, the distance between theshields (gap thickness) determines the bit length for the read sensor.With the ever increasing pressure to increase data capacity and datarate, engineers are constantly under pressure to decrease the bit length(and therefore gap thickness) of read sensors. One way to decrease thisgap thickness is by a partial mill process. Sensors have traditionallybeen constructed by depositing sensor layers as full film layers ontothe first gap layer. A photoresist mask is then formed over the areathat is to be the sensor and a material removal process such as ionmilling is performed to remove material from the areas not covered bythe mask. This material removal process has traditionally been performeddown into the first gap layer, removing a portion of the first gapmaterial at either side of the sensor.

Since this material removal process removes a portion of the first gaplayer, it has been necessary to deposit a thick first gap layer in orderprevent electrical shorts through the gap to the first shield. Such ashort would be a catastrophic event that would render the sensorunusable. In these prior art heads, hard bias layers, constructed of ahard magnetic material such as CoPtCr have then been deposited over thisetched out portion of the first gap layer at either side of the sensorto provide magnetic biasing to bias the magnetic moment of the freelayer in the desired direction parallel with the ABS.

As discussed above, the removal of a portion of the first gap duringformation of the sensor requires a larger overall gap thickness toprevent shorting. One way to overcome this is to use a partial millprocess in which the material removal process (ie. ion milling) used toconstruct the sensor is terminated before all of the sensor material hasbeen removed, such as when the material removal process has reached apoint around the AFM layer (usually PtMn) near the bottom of the sensor.By stopping the milling process within the sensor layers, such as at theAFM layer no gap material is removed. This allows a much thinner gap tobe deposited. The bias layers are then deposited on top of the remainingsensor layer rather than on the gap layer.

A problem that arises from such partial milling is that the bias layerproperties are different when deposited over the AFM layer or othersensor layer than they are when deposited over the gap layer. The gaplayer, usually Al₂O₃ is amorphous. It therefore has no crystallinestructure to impart to the seed layer or to the hard bias material whenthe material is deposited onto the gap. Therefore, a hard bias structuredeposited over the amorphous gap layer can exhibit a desired epitaxialgrowth that provides desired high retentive moment and high coercivityneeded for free layer biasing.

However, the AFM layer, such as for example PtMn, as well as othersensor layers are not amorphous and exhibit their own grain structures.When the hard bias layers are deposited over the AFM layer, the grainstructure of the underlying AFM layer carries through to the seed layerand hard bias layers. This grain structure being undesirable for optimalhard bias properties results in degraded biasing properties. This leadsto free layer instability and associated signal noise. For example,depositing a CoPtCr hard magnetic material with a Cr seed layer over aPtMn substrate results in a CoPtCr hard bias layer with a coercivity ofonly around 600 Oe. This is much lower than the roughly 1400 Oecoercivity obtained when the same hard bias layer and seed are depositedon a glass (amorphous) substrate.

Therefore there is a strong felt need for a hard bias structure that canbe formed over an AFM material or other sensor material while stillexhibiting the necessary hard magnetic properties needed for effectivefree layer biasing. Such a bias structure would preferably allow a hardbias layer such as one containing Co, Pt and Cr to be applied over anAFM layer such as PtMn whithout the hard bias layer taking on theundesirable grain structure of the underlying AFM layer.

SUMMARY OF THE INVENTION

The present invention provides a magnetoresistive sensor having improvedfree layer biasing. The sensor includes a layer having a crystallinestructure. This layer can be for example an antiferromagnetic (AFM)material such as PtMn. A seed layer that includes CrMo is deposited overthe crystalline layer. A layer of hard magnetic material is then formedover the seed layer. The hard magnetic material can be for exampleCoPtCr.

The novel seed layer structure of the present invention advantageouslyallows the hard magnetic layer deposited there over to exhibit excellenthard magnetic properties for stable and efficient free layer biasing.The seed layer interrupts the undesirable grain structure of theunderlying crystalline layer and initiates a desired crystalline growthin the hard bias layer deposited above.

In addition, the novel seed layer does not contain Ta and therefore doesnot experience the oxidation problems exhibited by prior art seedlayers. This allows the seed layer to remain in an un-oxidized state,providing better electrical conductivity at the junction of the sensorwith the bias layer and leads.

The seed layer can be a single layer of Si sandwiched between two layersof CrMo or could also be a layer of Si sandwiched between a layer of Crand a layer of CrMo.

These and other advantages and features of the present invention will beapparent upon reading the following detailed description in conjunctionwith the Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of thisinvention, as well as the preferred mode of use, reference should bemade to the following detailed description read in conjunction with theaccompanying drawings which are not to scale.

FIG. 1 is a schematic illustration of a disk drive system in which theinvention might be embodied;

FIG. 2 is an ABS view of a slider, taken from line 3-3 of FIG. 2,illustrating the location of a magnetic head thereon;

FIG. 3 is an ABS view of a magnetic sensor according to an embodiment ofthe present invention taken from circle 3 of FIG. 2; and

FIG. 4 is an expanded cross sectional view of a seed layer for use inthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The following description is of the best embodiments presentlycontemplated for carrying out this invention. This description is madefor the purpose of illustrating the general principles of this inventionand is not meant to limit the inventive concepts claimed herein.

Referring now to FIG. 1, there is shown a disk drive 100 embodying thisinvention. As shown in FIG. 1, at least one rotatable magnetic disk 112is supported on a spindle 114 and rotated by a disk drive motor 118. Themagnetic recording on each disk is in the form of annular patterns ofconcentric data tracks (not shown) on the magnetic disk 112.

At least one slider 113 is positioned near the magnetic disk 112, eachslider 113 supporting one or more magnetic head assemblies 121. As themagnetic disk rotates, slider 113 moves radially in and out over thedisk surface 122 so that the magnetic head assembly 121 may accessdifferent tracks of the magnetic disk where desired data are written.Each slider 113 is attached to an actuator arm 119 by way of asuspension 115. The suspension 115 provides a slight spring force whichbiases slider 113 against the disk surface 122. Each actuator arm 119 isattached to an actuator means 127. The actuator means 127 as shown inFIG. 1 may be a voice coil motor (VCM). The VCM comprises a coil movablewithin a fixed magnetic field, the direction and speed of the coilmovements being controlled by the motor current signals supplied bycontroller 129.

During operation of the disk storage system, the rotation of themagnetic disk 112 generates an air bearing between the slider 113 andthe disk surface 122 which exerts an upward force or lift on the slider.The air bearing thus counter-balances the slight spring force ofsuspension 115 and supports slider 113 off and slightly above the disksurface by a small, substantially constant spacing during normaloperation.

The various components of the disk storage system are controlled inoperation by control signals generated by control unit 129, such asaccess control signals and internal clock signals. Typically, thecontrol unit 129 comprises logic control circuits, storage means and amicroprocessor. The control unit 129 generates control signals tocontrol various system operations such as drive motor control signals online 123 and head position and seek control signals on line 128. Thecontrol signals on line 128 provide the desired current profiles tooptimally move and position slider 113 to the desired data track on disk112. Write and read signals are communicated to and from write and readheads 121 by way of recording channel 125.

With reference to FIG. 2, the orientation of the magnetic head 121 in aslider 113 can be seen in more detail. FIG. 3 is an ABS view of theslider 113, and as can be seen the magnetic head including an inductivewrite head and a read sensor, is located at a trailing edge of theslider. The above description of a typical magnetic disk storage system,and the accompanying illustration of FIG. 1 are for representationpurposes only. It should be apparent that disk storage systems maycontain a large number of disks and actuators, and each actuator maysupport a number of sliders.

With reference now to FIG. 3, a magnetoresistive sensor 300 according toan embodiment of the invention includes sensor stack 302 sandwichedbetween first and second gap layers 304, 306. The sensor stack 302includes a magnetic pinned layer structure 308 and a magnetic free layer310. A non-magnetic, electrically conductive spacer layer 312, such asCu, is sandwiched between the free layer 310 and the pinned layerstructure 308. A capping layer 314, such as Ta, may be provided at thetop of the sensor stack 302 to protect the sensor from damage duringmanufacturing, such as from corrosion during subsequent annealingprocesses.

The pinned layer 308 can be a simple pinned structure or an antiparallel(AP) pinned structure and is preferably an AP pinned structure includingfirst and second magnetic layers (AP1) 316, and (AP2) 318 which may befor example CoFe antiparallel coupled across a thin AP coupling layer320 such as Ru. The free layer 310 can be constructed of variousmagnetic materials such as NiFe or CoFe, and may include a layers ofCoFe and NiFe, preferably with a layer of CoFe or Co adjacent to thespacer 312 for optimal sensor performance.

As can be seen with reference to FIG. 3, the sensor stack 302 has firstand second laterally opposed side walls 322, 324 that define thetrack-width or active area of the sensor. A layer of antiferromagneticmaterial (AFM) 326, preferably PtMn, formed at the bottom of the sensorstack 302 is exchange coupled with the AP I layer 316. The AFM layer hasan extremely high coercivity, and when exchange coupled with the AP Ilayer 316 strongly pins the magnetic moment of the AP1 layer 316 asindicated by arrowhead 328. This in turn strongly pins the moment 330 ofthe AP2 layer 318 by antiparallel exchange coupling across the APcoupling layer 320. Also as can be seen, the AFM layer 326 has a portion332 that is disposed within the track width or active area of the sensor300, but also has first and second laterally extending portions 334, 336that extend laterally outward beyond the active area of the sensor 300.The laterally extending portions 334, 336 may be slightly thinner thanthe inner portion 332 due to a partial milling process used to constructthe sensor 300. It should be pointed out other sensor layers, such asthe pinned layer 308 or spacer 312 could also extend beyond the activearea of the sensor 300 as well. The extension of the AFM layer 326 aloneis by way of example only.

With continued reference to FIG. 3, the sensor 300 includes first andsecond hard magnetic, bias layers (HB layers) 338, 340. In addition,first and second leads 337, 339 are formed over the HB layers 338, 340.The leads 337, 339 may be constructed of, for example, Ta, Au, Rh orsome other electrically conducting material. The HB layers 338, 340 arepreferably constructed of an alloy containing Co, Pt and Cr, morespecifically Co₈₀ Pt₁₂, Cr₈. The hard bias layers 338, 340 have a highmagnetic coercivity, and are magnetostatically coupled with the freelayer 310 to bias the magnetic moment of the free layer 310 in adirection parallel with the ABS as indicated by arrow 341.

Seed layers 342, 344 are provided beneath the HB layers 338, 340. Theseed layers 342, 344 preferably extend over the laterally extendingportions 334, 336 of the AFM layer as well as over the side walls 322,324 of the sensor stack 302.

With reference to FIG. 4, the seed layers 342, 344 are each constructedas a composite of layers. In one possible embodiment the seed layers342, 344 each include a layer of Si 402 sandwiched between first andsecond layers of CrMo 404, 406. In another possible embodiment the seedlayers 342, 344 can be constructed as a layer of Si 402 sandwichedbetween a layer of CrMo 404 and a layer of Cr 406, with the layer ofCrMo 404 preferably being at the top (ie. closest to the hard biaslayers 338, 340).

With continued reference to FIG. 4, the layer 406 closest to the AFMlayer preferably has a thickness of 2 to 15 Angstroms, while the layer404 closest to the hard bias layer 336, 340 preferably has a thicknessof 40 to 60 Angstroms. The Si layer 402 preferably has a thickness of5-15 Angstroms. These dimension apply whether the seed layers 342, 244have the Cr/Si/CrMo construction or the CrMo/Si/CrMo construction.

The seed layers 342, 344 of the present invention provide an advantagewith regard to oxidation, compared with prior art seed layers, whichhave included a layer of Ta. Ta is a highly reactive metal, and theprior art seed layers, such as Ta/Si/CrMo or Ta/Si/Cr have been found tooxidize. The above described seed layers, which do not include Ta do nothave this corrosion problem.

The seed layers 338, 340 promote a desired epitaxial crystalline growthin the HB layers 338, 340 deposited thereon. This desired grainstructure in the hard bias layers 338, 340 allows the hard bias layersto have a desired high magnetic moment and high coercivity that isnecessary for strong free layer biasing.

The above described hard magnetic material (CoPtCr) has beendemonstrated to have a very high coercivity and squarenesss when used inconjunction with the above described seed layers 342, 344 and depositedover the crystalline AFM material 332. Although various compositions ofthe hard bias layer 338, 340 are possible, excellent results have beenachieved with Co₈₀Pt₁₂Cr₆.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Other embodiments falling within the scope of the inventionmay also become apparent to those skilled in the art. Thus, the breadthand scope of the invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

1. A magnetoresistive sensor, comprising: a sensor stack having firstand second laterally opposed sides defining an active area; a layer ofantiferomagnetic material having first and second laterally extendingportions that extend beyond the active area of the sensor, the laterallyextending portions each having a surface; a seed layer formed over thesurface of the each laterally extending portion of the layer ofantiferromagnetic material, the seed layer comprising a first layercomprising CrMo, a second layer comprising CrMo and a layer comprisingSi sandwiched between the first and second layers comprising CrMo; andfirst and second hard magnetic layers formed on the seed layer over eachof the first and second laterally extending portions of the layer ofantiferromagnetic material.
 4. A magnetoresistive sensor as in claim 1wherein the seed layer comprises a layer of CrMo having a thickness ofbetween 2 and 15 Angstroms.
 2. A magnetoresistive sensor as in claim 1,wherein: The first layer comprising CrMo has a thickness of between 2and 15 Angstroms; the second layer comprising CrMo has a thickness ofbetween 40 and 60 Angstroms; and the layer comprising Si has a thicknessof between 5 and 15 Angstroms.
 3. A magnetoresistive sensor as in claim1 wherein the seed layer extends over the first and second laterallyopposed sides of the sensor stack.
 4. A magnetoresistive sensor,comprising: a sensor stack having first and second laterally opposedsides defining an active area; a layer of antiferomagnetic layer havingfirst and second laterally extending portions that extend beyond theactive area of the sensor, the laterally extending portions each havinga surface; a seed layer formed over the surface of the each laterallyextending portion of the layer of antiferromagnetic material, the seedlayer comprising a layer comprising CrMo, a layer comprising Cr and alayer comprising Si sandwiched between the layer comprising CrMo and thelayer comprising Cr; and first and second hard magnetic layers formed onthe seed layer over each of the first and second laterally extendingportions of the layer of antiferromagnetic material.
 5. Amagnetoresistive sensor as in claim 4, wherein the seed layer comprises:the layer comprising Cr has a thickness of between 2 and 15 Angstroms;the layer comprising CrMo has a thickness of between 40 and 60Angstroms; and the layer comprising Si has a thickness of between 5 and15 Angstroms.
 6. A magnetorsistive sensor, comprising: a layer ofmaterial having a crystalline structure; a seed layer formed over thelayer having a crystalline structure, the seed layer comprising a firstlayer comprising CrMo, a second layer comprising CrMo and a layercomprising Si sandwiched between the first and second layers comprisingCrMo; and a hard magnetic material formed over the seed layer.
 7. Amagnetoresistive sensor as in claim 6, wherein the hard magneticmaterial comprises CoPtCr.
 8. A magnetoresistive sensor as in claim 6,wherein: the first layer comprising CrMo has a thickness of 2-15Angstroms and is disposed adjacent to the layer having a crystallinestructure; the second layer comprising CrMo has a thickness of 40-60Angstroms and is disposed adjacent to the hard magnetic material; andthe layer comprising Si has a thickness of 5-15 Angstroms.
 9. Amagnetorsistive sensor, comprising: a layer of material having acrystalline structure; a seed layer formed over the layer of materialhaving a crystalline structure, the seed layer comprising a layercomprising CrMo, a layer comprising Cr and a layer comprising Sisandwiched between the layer comprising CrMo and the layer comprisingCr; and a hard magnetic material formed over the seed layer.
 10. Amagnetoresistive sensor as in claim 9, wherein: the layer comprising Crhas a thickness of 2-15 Angstroms and is disposed adjacent to the layerhaving a crystalline structure; the layer comprising CrMo has athickness of 40-60 Angstroms and is disposed adjacent to the hardmagnetic material; and the layer comprising Si has a thickness of 5-15Angstroms.
 11. A magnetic data recording system, comprising: a magneticmedium; a slider; an actuator connected with the slider to move theslider adjacent to a surface of the magnetic medium; and amagnetoresistive sensor connected with the slider, the magnetoresistivesensor comprising: a layer of material having a crystalline structure; aseed layer formed over the layer of material having a crystallinestructure, the seed layer comprising a first layer comprising CrMo, asecond layer comprising CrMo and a layer comprising Si sandwichedbetween the first and second layers comprising CrMo; and a hard magneticmaterial formed over the seed layer.
 12. A magnetic data recordingsystem as in claim 1 1, wherein: the first layer comprising CrMo has athickness of 2-15 Angstroms and is disposed adjacent to the layer havinga crystalline structure; the second layer comprising CrMo has athickness of 40-60 Angstroms and is disposed adjacent to the hardmagnetic material; and the layer comprising Si has a thickness of 5-15Angstroms.
 13. A magnetic data recording system, comprising: a magneticmedium; a slider; an actuator connected with the slider to move theslider adjacent to a surface of the magnetic medium; and amagnetoresistive sensor connected with the slider, the magnetoresistivesensor comprising: a layer of material having a crystalline structure; aseed layer formed over the layer of material having a crystallinestructure, the seed layer comprising a layer comprising CrMo, a layercomprising Cr and a layer comprising Si sandwiched between the layercomprising CrMo and the layer comprising Cr; and a hard magneticmaterial formed over the seed layer.
 14. A data recording system as inclaim 13, wherein: the layer comprising Cr has a thickness of 2-15Angstroms and is disposed adjacent to the layer having a crystallinestructure; the layer comprising CrMo has a thickness of 40-60 Angstromsand is disposed adjacent to the hard magnetic material; and the layercomprising Si has a thickness of 5-15 Angstroms.
 15. A magnetoresistivesensor comprising: a plurality of sensor layers; a portion of theplurality of sensor layers terminating at first and second laterallyopposed sides to define an active sensor area; a portion of theplurality of sensor layers extending beyond the active area of thesensor to form first and second laterally extending portions; a seedlayer formed over the laterally extending portions of the sensor layers,the seed layer comprising a first layer comprising CrMo, a second layercomprising CrMo and a layer comprising Si sandwiched between the firstand second layers comprising CrMo; and a hard magnetic material formedover the seed layer over each of the first and second laterallyextending portions of the sensor layers.
 16. A sensor as in claim 15wherein the seed layer extends a least partially over at least one ofthe first and second laterally opposed sides of the sensor stack thatdefine the active sensor area.
 17. A sensor as in claim 15, wherein: thefirst layer comprising CrMo has a thickness of 2-15 Angstroms and isdisposed adjacent to the layer laterally extending portion of the sensorstack; the second layer comprising CrMo has a thickness of 40 to 60Angstroms and is disposed adjacent to the hard magnetic material; andthe layer comprising Si has a thickness of 5 to 15 Angstroms.
 18. Amagnetoresistive sensor comprising: a plurality of sensor layers; aportion of the plurality of sensor layers terminating at first andsecond laterally opposed sides to define an active sensor area; aportion of the plurality of sensor layers extending beyond the activearea of the sensor to form first and second laterally extendingportions; a seed layer formed over the laterally extending portions ofthe sensor layers, the seed layer comprising a layer comprising CrMo, alayer comprising Cr and a layer comprising Si sandwiched between thelayer comprising CrMo and the layer comprising Cr; and a hard magneticmaterial formed over the seed layer over each of the first and secondlaterally extending portions of the sensor layers.
 19. A sensor as inclaim 18 wherein the seed layer extends a least partially over at leastone of the first and second laterally opposed sides of the sensor stackthat define the active sensor area.
 20. A sensor as in claim 15,wherein: the layer comprising Cr has a thickness of 2-15 Angstroms andis disposed adjacent to the layer laterally extending portion of thesensor stack; the layer comprising CrMo has a thickness of 40 to 60Angstroms and is disposed adjacent to the hard magnetic material; andthe layer comprising Si has a thickness of 5 to 15 Angstroms.